1. Field of the Invention
The present invention relates to a high speed switching current source or "charge pump" which operates as a current source to provide a charging current and as a current sink to discharge current. Typically, the output of the charge pump is connected to a capacitor which is charged and discharged by the charge pump in order to achieve a desired voltage level across the capacitor. In a typical application, the charge pump is used in a phase-locked loop system including a charge pump, low pass filter, voltage controller oscillator and phase detector. The phase detector compares the phase of an input signal with that of an output signal and generates a "charge up" or "charge down" signal to the charge pump depending upon the phase error. Current flow at the output of the charge pump is rapidly switched to provide either a current source or a current sink to charge or discharge a capacitor which is part of the low pass filter. The voltage across the capacitor is applied to the voltage controlled oscillator to adjust the frequency until the output of the VCO is phase and frequency locked to the input signal.
In order to achieve proper charging and discharging of the capacitor, the charge pump must have the ability to rapidly switch the charge or discharge current on and off. Otherwise, the rate of charging or discharging of the capacitor will not be constant. Thus, it is desirable that the output characteristics of the charge pump will be such that the current output is a step function so that the capacitor will charge and discharge at a fixed rate.
2. Description of the Prior Art
Typically, a charge pump includes output current transistors to provide the desired current sink or source operation. Additional switching transistors or transfer gates are located in the output current path and are switched to couple the desired output transistor to the output terminal of the charge pump. This introduces both parasitic resistance and capacitance, thus slowing the switching speed of the device. In addition, the large transfer gates need to charge up to the full power supply level, thus further slowing down the turn on time.
A phase-locked loop system incorporating a charge pump is illustrated in U.S. Pat. No. 4,208,075 to Wessler. The charge pump generates a positive or negative phase difference signal, which represents operation as a current sink or source, which is applied to a low pass filter circuit. Switched current sources are also disclosed in U.S. Pat. Nos. 3,886,485 to Takahashi and 3,904,988 to Hsiao. These patents are directed to voltage controlled oscillators which include a timing capacitor charged and discharged by a current source generator which may be switched to operate as a sink or source. In the Hsiao patent in particular, the switching means used to select current sink or source operation is located in the output current path and thus is subject to the disadvantages described above. Although these circuits are acceptable for many applications, faster switching speeds are necessary for more critical applications. In particular, phase-locked loops employed in data separators or Winchester discs operate at approximately 10 MHz and therefore require extremely fast switching charge pumps. The present invention is directed to a charge pump capable of providing the high switching speed necessary for such applications.